The ordination and classification of macroinvertebrate assemblages in the catchment of the River Wye in relation to environmental factors

SUMMARY. 1. Macroinvertebrates were sampled from three habitats at forty-five sites in the catchment of the Welsh River Wye. Species assemblages were ordinated by DECORANA. classified by TWIN-SPAN and related to physico-chemical factors using correlation and multiple discriminant analysis respectively.
2. DECORANA axis 1 was correlated with pH or total hardness, whilst axis 2 correlated with slope or distance from source. TWINSPAN groupings were also related to hardness and, to a lesser extent, slope. Assemblages at soft-water sites (<15gm^?3 CaCO₃) were composed mostly of Plecoptera but at hard-water sites, even at high slope (>10 m km^?1), the fauna was dominated numerically by Ephemerop-tera, net-spinning Trichoptera, Mollusca and Crustacea.
3. We suggest that our data do not support the River Continuum Concept unless there are modifications to allow for multiple gradients.